Since the late 1970s, waveguides have increasingly supplanted conventional signal lines for transmitting information. Rather than encoding information in electrical signals and transmitting the encoded electrical signals via signal lines, the same information can be encoded in a channel of electromagnetic radiation and transmitted via waveguides, such as optical fibers, ridge waveguides, and photonic crystal waveguides. The term “channel,” also called “optical channel,” refers to electromagnetic radiation transmitted at one wavelength through a waveguide. Transmitting information encoded in channels via waveguides has a number of advantages over transmitting encoded electrical signals via signal lines. First, degradation or loss is much less for channels transmitted via waveguides than for electrical signals transmitted via signal lines. Second, waveguides can be fabricated to support a much higher bandwidth than signal lines. For example, a single copper or aluminum wire can only transmit a single electrical signal, while a single optical fiber can be configured to transmit about 100 or more channels. Finally, electromagnetic radiation provides, in general, a much higher transmission rate.
Recently technological advances in semiconductor fabrication techniques have made it possible to fabricate waveguides that can be integrated with electronic devices, such as memory and processors, to form photonic integrated circuits (“PICs”), where the waveguides may be used to transmit information encoded in channels between the electronic devices. PICs are the photonic equivalent of electronic integrated circuits and may be implemented on a small substrate of semiconductor material that forms the base of the electronic devices.
In order to effectively implement PICs, passive and active photonic components are needed. Waveguides, attenuators, and filters are examples of passive photonic components that can be fabricated using conventional epitaxial and lithographic methods and may be used to direct the propagation of channels between electronic devices and filter channels. Active photonic components include electro-optic modulators. Accordingly, physicists, engineers, and computer scientists continue to seek improved active and passive photonic components for use in PICs or other photonic systems.